FIG. 1 is a configuration diagram of a general magnetic resonance wireless power transmission system.
Referring to FIG. 1, a magnetic resonance wireless power transmission system 1 includes a power transmitting unit (PTU) 10 that supplies a power signal through magnetic resonance and a power receiving unit (PRU) 12 that receives the power signal from the PTU 10.
The PTU 10 includes a power amplifier 100 and a resonator 110. The power amplifier 100 includes N-channel metal oxide semiconductors (NMOSs) M1 101 and M2 102, and the resonator 110 includes a capacitor (Cs) 111 and an inductor (L) 112. In FIG. 1, the power amplifier 100 is limited to class-D, but may be replaced by a class-AB or class-B power amplifier. The power amplifier is operated at a driving frequency (fdrv) 103. Therefore, an output of the power amplifier 100 composed of M1 101 and M2 102 is in the form of a square wave which varies between a supply voltage (VSUP) 104 and a ground voltage 105 at the driving frequency (fdrv) 103. According to alliance for wireless power (A4WP), which is a resonant wireless power transfer standard, the driving frequency fdry is set to 6.78 MHz. The output of the power amplifier 100 is applied to the resonator 110 composed of the capacitor (Cs) 111 and the inductor (L) 112. Here, L 112 denotes an 101 equivalent inductance of a transmission (TX) antenna, and Resr 113 is a parasitic resistance component of the antenna. A resonant frequency fR,PTU of the PTU resonator 110 is shown in Expression 1.
                              f                      R            ,            PTU                          =                  1                      2            ⁢            π            ⁢                                          L                ·                Cs                                                                        [                  Expression          ⁢                                          ⁢          1                ]            
In general, the resonant frequency fR,PTU is controlled to be identical to the driving frequency fdrv, and in some cases, the resonant frequency fR,PTU is slightly lower than the driving frequency fdrv to increase efficiency of the power amplifier 100. Since the transistors M1 101 and M2 102 can perform zero-voltage switching (ZVS) when this condition is satisfied, switching loss can be remarkably reduced.
Meanwhile, the PRU 12 that receives a wireless power signal includes a resonator 120 composed of a capacitor (Cs1) 122 and an inductor (L1) 124 serving as an antenna, a rectifier 130 composed of diodes D1 to D4, and a direct current (DC)-DC converter 140. Since an output of the rectifier 130 is a rectified voltage, a capacitor CRECT is used to convert the rectified voltage into a DC voltage. After a DC voltage VRECT generated by the capacitor CRECT is converted into a suitable voltage for a load 150 using the DC-DC converter 140, the load 150 is operated. The DC-DC converter 140 may employ a linear low dropout (LDO) regulator, a switching converter, a charge pump, etc., but is not limited thereto. As shown in FIG. 1, the rectifier 130 may be a full-wave rectifier, but can also be implemented using a half-wave rectifier. As shown in FIG. 1, the rectifier 130 may be implemented using the passive-device diodes D1 to D4, but can also be implemented as an active rectifier using an active device.
A resonant frequency of the PRU resonator 120 is determined as shown in Expression 2.
                              f                      R            ,            PRU                          =                  1                      2            ⁢            π            ⁢                                                                                L                    ⁢                    1                                    ·                  Cs                                ⁢                                                                  ⁢                1                                                                        [                  Expression          ⁢                                          ⁢          2                ]            
When the resonators 110 and 120 of the PTU 10 and the PRU 12 have the same resonant frequency and the two antennas 112 and 124 are close to each other, magnetic resonance occurs. At this time, energy is transferred from the antenna 112 of the PTU to the antenna 124 of the PRU.
Since energy is not effectively transferred when resonant frequencies of the resonators 110 and 120 differ from each other, it is very important to match the resonant frequencies of the resonators 110 and 120 the same. A method of tuning passive devices, an inductor (L) and a capacitor (C) is generally used to match resonance characteristics of the PTU 10 and the PRU 12. However, since the method of tuning passive devices involves physically tuning L and C, productivity is low, and it is not easy to cope with a case in which L and C values are changed due to an external factor.
In terms of efficiency, it is most efficient for the PTU 10 to transmit as much power as is required by the PRU 12. However, when too much energy is transmitted, the voltage VRECT of the PRU 12 increases excessively and destroys the rectifier 130 and the DC-DC converter 140, and when too little energy is transmitted, it is not possible to supply a desired power to the load 150. Therefore, the PTU 10 receives feedback with a requirement of the PRU 12 and controls the output power. According to A4WP, the PTU 10 and the PRU 12 communicate with each other using Bluetooth communication. There are generally three methods by which the PTU 10 controls power.
(1) Driving frequency control
(2) Power amplifier burst switching control
(3) Power amplifier supply voltage control
Driving frequency control involves changing the driving frequency of the power amplifier 100. Since energy supplied to the PTU resonator 110 can be changed, power control is possible. This method is used by a resonant inverter, Qi and power matters alliance (PMA), which are inductive wireless power transfer methods. However, according to standards such as A4WP, since a driving frequency is fixed, such control is difficult.
Power amplifier burst switching control involves controlling an average power applied to the resonator 110 by operating or not operating the power amplifier 100. This may be referred to as a burst switching operation. This method is used to transmit power according to near field communication (NFC) and so on. Since a frequency spectrum is generated in a form in which the driving frequency of the power amplifier 100 and a burst frequency are modulated in this method, it is possible to think that a radiated frequency is generated with a predetermined width. Since a frequency permitted in A4WP is about 6.78±15 kHz, such control is possible, but it is necessary to prevent the frequency from exceeding the bandwidth.
FIG. 2 is a configuration diagram of a magnetic resonance wireless power transmission system using power amplifier supply voltage control.
Referring to FIG. 2, it is possible to adjust energy supplied to a resonator 260 by controlling a supply voltage (VSUP) 250 of a power amplifier 220 with a DC-DC converter 240 positioned between a supply 200 and the power amplifier 220. Since the above-described method can be used even when a structure of the power amplifier 220 is not class-D, the above-described method is the most flexible method. However, additional cost is required to configure the DC-DC converter 240, and there is a risk that loss of the DC-DC converter 240 will reduce the overall efficiency of the wireless power transmission system.
FIG. 3 is a circuit diagram of a wireless power transfer unit that controls supply of power by controlling a switch device.
Referring to FIG. 3, after energy is transferred to a resonant tank using an inductor 300 and a capacitor 310 and converted into a DC through diodes 320 and 330, the energy is supplied to a load 360 using a control circuit 340. In this case, the control circuit 340 controls the switch device 350, thereby controlling power supplied to the load 360.
FIG. 4 is a configuration diagram of a wireless power transfer unit using a resonant frequency control method employing a clock signal.
Referring to FIG. 4, N1 410 is operated as a resistance so that Vout 400 becomes a desired voltage, or N1 410 is controlled as a switch using a clock signal 420 to control Vout 400. According to this method, a resonant frequency of a resonator is changed to adjust energy received by a resonant tank.